The present invention relates to blue colored glass compositions. More particularly, the present invention relates to blue colored glasses which can be made by the float process and which are primarily, but not exclusively, intended for use in architectural applications.
It has become desirable that glasses primarily intended for use in architectural applications, e.g. glazings for buildings, should be heat absorbing. A useful colorant for such purposes is iron because the presence of ferrous, as distinct from ferric, iron in the glass reduces the infrared transmission of the glass. Iron provides the glass with a green coloration. However, a blue colored glass would in some instances be preferred by architects.
Blue colored glasses are most easily produced simply by adding cobalt oxide to a batch composition. However, cobalt oxide reduces the light transmission of the glass and does relatively little to reduce the infrared (heat) transmission.
Alternatively, blue glasses can be produced by using solely iron as the colorant. This is achieved by increasing the proportion of total iron in the ferrous state to very high levels or values, often in excess of 50%. However, the glass then becomes progressively more difficult to melt without the risk of inclusions being formed in the glass, particularly silica scum. Furthermore, sulphate is often used to refine the glass and, at very high ferrous levels, iron sulphide may be produced which makes the predominant color of the glass yellow or amber.
Accordingly, to overcome these problems, batches containing very low amounts of sulphate and techniques such as vacuum refining have been suggested. It has been reported that this latter procedure is, in itself, difficult to operate. Alternatively, special reagents, which are relatively expensive, may be used to avoid the need for special refining techniques. For example, oxides of tin may be required. Ultraviolet radiation absorption is, however, reduced in glasses having a high ferrous to ferric iron ratio. This is because ferric iron absorbs strongly in the ultraviolet portion of the spectrum.